The present invention relates to a brake fluid pressure retaining unit for retaining brake fluid pressure within a wheel cylinder after releasing a brake pedal. The brake fluid pressure retaining unit includes a solenoid valve arranged in a brake fluid passage between a master cylinder and a wheel cylinder so as to shut off the brake fluid passage.
A brake fluid pressure retaining unit is known for facilitating a smooth starting operation on a slope without any unintentional backward displacement of a vehicle (for example Japanese Laid-open Patent Publication Nos. 63-43854 and 9-202159). In such a brake fluid pressure retaining unit, braking force continues to act on the vehicle after releasing the brake pedal until driving force is exerted on the vehicle.
In this known brake fluid pressure retaining unit, braking force is exerted on the vehicle without requiring driver""s operation. However, it is preferable that the brake fluid pressure retaining unit merely works when necessary such that the driver does not experience awkward driving operation of the vehicle. Specifically, the brake fluid pressure retaining unit may be actuated when the transmission of the vehicle selects a driving position. When a non-driving position is selected and the vehicle does not start on a slope, the brake fluid pressure retaining unit does not have to be actuated. Accordingly, the brake fluid pressure retaining unit may be controlled so as to release retained brake fluid pressure when the transmission selects a non-driving position.
However, a problem occurs in a conventional brake fluid pressure retaining unit RUxe2x80x2 shown in FIGS. 14A and 14B if the brake fluid pressure to be retained is released immediately after the transmission selects a non-driving position. The brake fluid pressure retaining unit RUxe2x80x2 includes a solenoid valve SV arranged in a brake fluid passage FP between a master cylinder MC and a wheel cylinder WC so as to shut off the brake fluid passage FP for retaining brake fluid pressure within the wheel cylinder WC as instructed by a control unit CUxe2x80x2. Such a brake fluid pressure retaining unit is known for example from Japanese Laid-open Patent Publication Nos. 60-12360 and 63-43854.
As shown in FIG. 14A, when the brake fluid pressure retaining unit RUxe2x80x2 is actuated, the solenoid valve SV shuts off the brake fluid passage FP for retaining brake fluid pressure within the wheel cylinder WC. When the driver shifts the transmission from a driving position to a non-driving position, the solenoid valve SV is switched to a communicating position where the solenoid valve SV allows communication of the brake fluid passage FP, thereby communicating the master cylinder MC and the wheel cylinder WC as shown in FIG. 14B. No problem occurs if the transmission is shifted to a non-driving position (1) when the driver does not depress the brake pedal BP or (2) while the driver depresses the brake pedal BP, i.e. pressure difference between the master cylinder MC and the wheel cylinder WC is small.
However, if the transmission is shifted to a non-driving position while the driver is loosening the brake pedal BP, impact is transmitted to the driver""s foot F due to fluctuation of brake fluid pressure upon communication of the brake fluid passage FP (FIG. 14B).
When the driver loosens the brake pedal BP while the solenoid valve SV is at a shut-off position where the solenoid valve SV shuts off the brake fluid passage FP, brake fluid pressure within the master cylinder MC decreases by way of resilient action of a return spring MCS. Meanwhile, brake fluid pressure within the wheel cylinder WC is retained since the solenoid valve SV is at the shut-off position. Therefore, pressure difference is made between the wheel cylinder WC and the master cylinder MC. If the driver shifts the transmission from a driving position to a non-driving position before leaving his foot from the brake pedal BP, the solenoid valve SV is switched to the communicating position and greater brake fluid pressure at the wheel cylinder WC is instantly transferred to the master cylinder MC. This creates pressure fluctuation, which gives impact to the driver""s foot F placed on the brake pedal BP.
Accordingly, the object of the invention is to eliminate the above drawback.
With the aforementioned drawback in view, the present invention seeks to provide a brake fluid pressure retaining unit for a vehicle, which comprises a solenoid valve arranged in a brake fluid passage between a master cylinder and a wheel cylinder and switchable between a communicating position where the brake fluid passage is communicating and a shut-off position where the brake fluid passage is shut off for retaining brake fluid pressure within the wheel cylinder, the solenoid valve being switched from the communicating position to the shut-off position when the vehicle stops with a brake pedal depressed on condition that a transmission of the vehicle is selected to a driving position, and the solenoid valve being returned to the communicating position when driving force is exerted on the vehicle, and brake fluid pressure retained within the wheel cylinder being kept until driving force is exerted on the vehicle such that braking force continues to act on the vehicle after releasing the brake pedal, wherein the brake fluid pressure retaining unit further comprises means for controlling said solenoid valve such that when the solenoid valve is in the shut-off position and the transmission is shifted from a driving position to a non-driving position, the solenoid valve is returned to the communicating position on condition that depression of the brake pedal is released.
According to the present invention, when the transmission is shifted from a driving position to a non-driving position with the solenoid valve switched to the shut-off position, the solenoid valve is returned to the communicating position on condition that depression of the brake pedal is released.
With such construction, even if the transmission is shifted from a driving position to a non-driving position, the solenoid valve is still in the shut-off position unless the driver releases the brake pedal. The solenoid valve is returned to the communicating position after depression of the brake pedal is released.
If depression of the brake pedal is released, the driver""s foot is not placed on the brake pedal, or otherwise the brake pedal is not returned further notwithstanding the driver places his foot on the brake pedal.
The term xe2x80x9cdriving positionxe2x80x9d in the transmission refers to a running range such as xe2x80x9cD rangexe2x80x9d in the case of automatic transmission and a starting gear such as xe2x80x9cLow speed gearxe2x80x9d in the case of manual transmission. Also, the term xe2x80x9cnon-driving positionxe2x80x9d in the transmission refers to a non-driving range such as xe2x80x9cN rangexe2x80x9d or xe2x80x9cP rangexe2x80x9d in the case of automatic transmission and xe2x80x9cNeutralxe2x80x9d in the case of manual transmission.
The wording xe2x80x9cwhen driving force is exerted on the vehiclexe2x80x9d means that output of a motor such as an engine and an electric motor is transmitted to wheels of the vehicle, and the vehicle is ready to start on an up slope with such driving force thus generated. Specifically, it means that (1) in the case of a manual transmission type vehicle, the time when the driver engages the clutch, (2) in the case of an automatic transmission type vehicle, the time when the driver depresses the accelerator pedal or (3) in the case of an automatic transmission type vehicle, in which driving force is automatically increased in accordance with releasing of the brake pedal so as to resist a slope, the time when the increment of the driving force is achieved (for example when the vehicle reaches to a xe2x80x9cstrong creep conditionxe2x80x9d such as described in Preferred embodiments of the present invention).
Another object of the invention is to provide a brake fluid pressure retaining unit for a vehicle, which comprises a solenoid valve arranged in a brake fluid passage between a master cylinder and a wheel cylinder and switchable between a communicating position where the brake fluid passage is communicating and a shut-off position where the brake fluid passage is shut off for retaining brake fluid pressure within the wheel cylinder, the solenoid valve being switched from the communicating position to the shut-off position when the vehicle stops with a brake pedal depressed on condition that a transmission of the vehicle is selected to a driving position, and the solenoid valve being returned to the communicating position when driving force is exerted on the vehicle, and brake fluid pressure retained within the wheel cylinder being kept until driving force is exerted on the vehicle such that braking force continues to act on the vehicle after releasing the brake pedal, wherein said brake fluid pressure retaining unit is controlled such that when the solenoid valve is in the shut-off position and the transmission is shifted from a driving position to a non-driving position, the solenoid valve is returned to the communicating position on condition that depression of the brake pedal is released.